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Turkey on Sunday officially recognised Russia's attacks on Ukraine as a "state of war" and said it was implementing an international treaty giving it the power to limit warships' passage through the strategic Dardanelles and Bosphorus straits.
"It's no longer a military operation but a state of war," Foreign Minister Mevlut Cavusoglu said.
"In this case, of course, whichever of them, it can be Russia or Ukraine, we apply the Montreux agreement."
The 1936 Montreux Convention governs the free movement of commercial ships in peacetime through the Bosphorus and the Dardanelles straits and grants Turkey the right to block warships in wartime if threatened.
Ukraine has officially asked Turkey to close the Dardanelles strait, and thus access to the Black Sea, to Russian ships.
Nato member Turkey, which has strong ties with Russia and Ukraine, did not immediately respond to this request.
But Mr Cavusoglu said Turkey could not prevent Russian and Ukrainian access to the Black Sea because of Article 19 of the convention, which allows littoral states' vessels to return to their home base.
He said whether the ship was registered to the home base was clear and warned against any abuse.
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"It should not be involved in the war after declaring it will return to the base and passing through the Bosphorus," Mr Cavusolglu said.
Turkey has offered to host talks between leaders of the two countries.
"We have repeatedly called for a diplomatic solution but the war is a reality right now," President Recep Tayyip Erdogan's top press aide, Fahrettin Altun, tweeted on Sunday.
